Bug 18127: use $GITIAN_TAG in fetch-inputs.sh

git remote set-url origin https://git.torproject.org/builders/gitian-builder.git

git fetch origin

git fetch --tags origin # XXX - why do we fetch tags specifically?

git checkout tor-browser-builder-3

git merge origin/tor-browser-builder-3

git checkout "$GITIAN_TAG"

cd inputs

while read dir url tag; do